The clinical application of PDT encompasses a large parameter space with multiple variable. It is not feasible to directly examine these variables in patients, particularly in cutaneous lesions where high response rates and long times to recurrence give delayed information. We hypothesize that three surrogate variables will allow us to obtain real- time and short-term information and will permit tailoring the therapy to individual patients and lesions. Aim 1: To investigate and validate in cells, animal models and patients, a set of surrogate variables that enable us to follow the course of PDT treatment, to predict the ultimate outcome, and to rationally optimize treatment parameters. The variables are photobleaching kinetics, epidermal toxic reaction, and minimal phototoxic light dose. We hypothesize that the interval between ALA application and irradiation has significant effects on clinical outcomes with alterations in efficacy, efficiency and selectivity. To evaluate these issues the second aim is: Aim 2. To examine the effects of the interval between ALA application and irradiation, utilizing both surrogate variables and conventional therapeutic endpoints to select treatment parameters and follow responses in two cutaneous diseases where the targets are keratinocytes or immune cells. Mechanistic understanding of clinical outcomes requires tissue-level information on photosensitizer distribution, PDT damage sites, and host responses. We will collect this information in Aim 3. Aim 3. In support of Aims 1-2 and in collaboration with Projects 1-3; to examine the distribution of photosensitizer localization and of putative photosensitizer receptors in carcinomas and CL; and the effects of sensitizers and PDT on skin-associated immune cells and cytokine synthesis.